Circuit interrupters such as circuit breakers are widely used in residential and industrial applications for the interruption of electrical current in power lines upon the occurrence of various fault conditions such as severe overcurrent caused by short circuits or by ground faults. A ground fault circuit interrupter (GFCI) is one which interrupts a circuit if it detects a leakage current to ground, usually called a ground fault. GFCIs are often provided in household type electrical receptacles which are often mounted in the home in places like bathrooms and kitchens to protect against such short circuits or leakage to ground due to water or moisture or the like entering the protective circuit or an appliance or device connected to the protective circuit. However, the GFCI circuit generally requires that it be coupled in a circuit protected by a circuit breaker in order to protect the circuit from overload and short circuit conditions. That is, an ordinary circuit breaker generally provides these overload and short circuit functions in connection with a GFCI receptacle.
Another type of electrical fault, namely arcing faults, also may occur in circuits which are protected by both GFCI receptacles and circuit breakers. These arcing faults are unintended arcs either from line to line or from line to ground. While conventional circuit breakers may detect relatively high level currents or voltages and interrupt the circuit, they generally cannot detect relatively low-level arcs. However, many low-level arcs should also be protected against. Generally speaking, there is existing technology for protecting against arcing faults, namely arc fault circuit interrupter (AFCI) products, such as those available from Square D Company, the assignee of this application.
Another problem associated with the process of interruption of the current during severe overcurrent conditions is arcing. Arcing occurs between the contacts of circuit breakers and/or ground fault interrupters used to interrupt the current, and is undesirable for several reasons. Arcing can cause deterioration of the contacts of the breaker and, can cause gas pressure to build up. It also necessitates circuit breakers and/or GFCIs with larger separation between the contacts in the open position to ensure that the arc does not persist with the contacts in the fully open position.
Prior art devices have used a number of approaches to limit the occurrence of arcing. In heavy duty switchgear, the circuit breaker contacts may be enclosed in a vacuum or in an atmosphere of SF.sub.6. Both of these approaches are expensive. Besides, SF.sub.6 has been identified as a greenhouse gas.
Another approach that has been used in circuit breakers to limit the amount of arcing is the use of a resistor connected in parallel with the main contacts of the circuit breaker. Upon opening of the main contacts, current can still flow through the shunt resistor, effectively reducing the amount of arcing in the main contacts. The current flowing through the resistor is less than the short circuit current that would flow through the main contacts in the absence of the resistor, and the opening of a second pair of contacts connected in series with the resistor can be accomplished with less arcing that would occur in the absence of the shunt resistor.
Khalid (U.S. Pat. No. 4,070,641) teaches a current limiting circuit breaker in which the current limiting contacts are in series with the main contacts of a breaker. Opening of the limiting contacts shunts high fault current through the resistor. The resistor is an iron wire resistor with a positive temperature coefficient (PTC) of resistance. The flow of the short circuit current through the resistor heats the resistor thereby increasing its resistance and limiting the buildup of the short circuit current.
Perkins et al. ("IEEE Transactions on Components, Hybrids, and Manufacturing Technology," CHMT-5, A New PTC Resistor For Power Applications, pgs. 225-230, June 1982) describes a PTC resistor that utilizes the metal-insulator solid state transition in (V, Cr).sub.2 O.sub.3. At a transition temperature of 80.degree. C., the resistivity of a ceramic body including (V, Cr).sub.2 O.sub.3 increases to a value 100 times the value at 20.degree. C. They disclose the use of a PTC element for overcurrent protection as a substitute for a bimetallic strip for overcurrent protection. The switch is connected to a PTC element that is shunted by the actuating coil for the switch. During normal operating conditions, the current flows through the PTC resistor. During short circuit conditions, the rapid heating of the PTC resistor leads to an increased resistance and voltage across the PTC resistor, diverting current through the actuating coil which then trips the switch.
Hansson et al. (U.S. Pat. No. 5,382,938) discloses a PTC element that is capable of withstanding short circuit currents without damage, thereby enabling it to be reused, as an overcurrent protection device for a motor. The PTC element is connected in series with a switch and in parallel with an excitation coil that operates the switch. An overcurrent in the circuit heats the PTC element and at a certain temperature, its resistance rises sharply. The voltage across the PTC element is then sufficient to cause the excitation coil to trip the switch. Hansson et al. (WO 91/12643) discloses a more complicated invention for motor and short circuit protection using a PTC element. A switch is connected in series with a tripping circuit consisting of two parallel connected current branches. One of these branches has the excitation coil for the switch while the other branch has two PTC resistors. Overcurrent conditions cause a buildup of voltage across the PTC resistors that then activates the excitation coil for the switch.
Chen (U.S. Pat. No. 5,629,658) discloses a number of devices in which PTC elements are used in conjunction with two or more switches to limit the current under short circuit conditions and thereby reduce the associated arcing. Chen uses a PTC element in a circuit breaker for absorbing the so-called "interruption" energy which could otherwise produce arcing during breaker operation.
Legatti (U.S. Pat. No. 4,931,894) is directed to a ground fault interrupter (GFCI) circuit which utilizes a GFCI differential transformer. A secondary winding is provided on the core of the GFCI transformer for producing a detectable signal in response to an arcing current between a power line and the grounded metal sheath or cover of a power cable, by connecting the secondary winding in series between the metal sheath and the neutral line. Among other things, this circuit requires the provision of a separate ground line and a separate wire for connection to the cable sheath.
Yet another approach used to reduce arcing in circuit breakers involves the use of mechanical means to break the arc. Belbel et al. (U.S. Pat. No. 4,562,323) discloses a switch in which an electrically insulating screen is inserted between the contacts during the opening of the contacts. The control of the movement of the screen is obtained by propulsion means separate from those causing the separation of the contacts. Belbel et al. (U.S. Pat. No. 4,677,266) discloses another switch that has an insulating screen that adapts the breaking speed as the current increases. Brakowski et al. (U.S. Pat. No. 4,801,772) discloses a current limiting circuit interrupter in which an insulating wedge is inserted between the contact arms as they open.
Most of the prior art methods discussed above are addressed towards industrial applications. For residential use, even though the voltage and the loads are smaller, commercially available circuit breakers and GFCIs may still have a significant amount of arcing accompanying their operation. The present invention achieves interruption of electrical current with a reduction in arcing, noise and gas venting. The present invention combines an AFCI and GFCI with PTC elements and related circuits for short circuit and overload protection in a convenient form suitable for residential use in a household electrical receptacle or the like. The present invention also reduces the cost and enclosure requirements for residential circuit protection.